Over 2000 nuclear tests took place between 1945 and the signing of the Comprehensive Nuclear Test Ban Treaty in 1996, with the United States and Soviet Union combining for over 1700 explosions.

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In signing the treaty, the International Monitoring System put in place by the United Nations keeps watch for clandestine nuclear explosions in four ways. Let's take a look at four methods that the U.N. uses to monitor secret nuclear tests — along with one interesting application of an everyday commercial tool to pinpoint the location of a nuclear detonation.

Forensic seismology
Most explosive tests after 1962 took place underground, in order to hide testing activity. Luckily, we already have a worldwide system designed to detect earthquakes and low level seismic activity, and this can double as a nuclear detonation detection system. The shock waves emanating from nuclear tests are quite strong – a 2009 North Korean underground nuclear test registered as a 4.52 on the Richter scale. Constant monitoring of the seismic activity allows for a quick set of initial data and paves the way for further investigation.

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Hydroacoustics
As you know from playing in a pool, water conducts sound, providing a wonderful (and plentiful) material to allow sound to propagate throughout the world with ease. Eleven monitoring stations are placed on strategically chosen areas in the Pacific, Atlantic, and Indian Ocean. The the stations are located on islands in the middle of a large body of water or ashore, with microphones placed in the water and communicating data back to continental stations.

Infrasound
Sound plays another role in detection, because large chemical and nuclear explosions create low frequency sound waves that continue through the atmosphere above the surface of the Earth. These sound waves are below the threshold of human hearing (17 Hz and less), with sixty stations across the globe monitoring these low frequency sound waves.

Radionuclide detection
One of the best (and oldest) ways to detect clandestine nuclear operations is through the monitoring of radioactive chemicals in the air. In the 1949, the United States used a series of weather balloons to collect atmospheric data in the Soviet Union, allowing the Truman administration to learn of Soviet nuclear capabilities, but weeks after the initial detonation.

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Radionuclide detection is far more sophisticated now, with 80 monitoring stations spread through the atmosphere and detecting the presence of radioactive isotopes in real time. Half of these stations are equipped to monitor for noble gases, particular Xenon; helpful in detecting underground nuclear tests as isotopes of Xenon and Argon find their way into the atmosphere over time.

Using the Global Positioning System
The same ethereal voice that tells your grandparents how to navigate through downtown also detects clandestine nuclear detonations. Not exactly a part of the International Monitoring System, but still playing an important role, is the existing system of commercial global positioning satellites spread throughout the world.

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The aforementioned 2009 detonation by North Korea took place one kilometer below the surface in order to avoid detection, as this depth is likely too far below the surface for radioactive material to pass to the air.

Although the explosion took place deep below the surface of the Earth, a shock wave moving and out from the place of detonation emanated into the atmosphere. Six GPS receivers in the area belonging to South Korea and another five scattered across Asia observed a spike in atmospheric electron density after the North Korean nuclear test. An increase in density decreases the speed at which signals travel to global positioning satellites. The shock wave after to 2009 North Korean test moved toward the 11 receivers at 540 miles per hour, allowing observers to trace the explosion to P'unggye, also the site with the highest observed seismic activity.

Keeping tabs
Over 270 stations organized by the International Monitoring System spread across the world monitor sound waves, seismic activity, and the presence of radionuclides. The member countries signing the Comprehensive Nuclear Test Ban Treaty maintain the stations and keep a distributed worldwide presence, making the nations aware of nuclear tests taking place anywhere in the world.